Design and function
Control unit
As on previous models, the control unit is located at the front of the luggage compartment on the right-hand side. The control unit analyses the incoming data from the antenna and forwards the relevant information to the instrument cluster. Since the data is transmitted via cable from the central antenna, the control unit is designed to pick up both frequencies (433/315 MHz). If a new control unit is installed, it must be coded accordingly.
Triggers (trigger senders) The four triggers, which are located under the wheel housing liners in each of the four wheel housings, send a 125 kHz signal directly to the wheel electronics units in order to transmit the desired information to a central antenna immediately.
When the vehicle is unlocked, the control unit initiates the first 125 kHz signal for each trigger in the four wheel housings one after the other, starting at the left front in clockwise direction. Then, the wheel electronics units are only triggered approx. every 60 seconds while the vehicle is moving. Since the range of the trigger signals is limited to the relevant wheel housing, any possibility of cross-talk affecting other wheels is almost totally eliminated. Depending on many and varied influences from the immediate environment, such as reflections (wet roads, metallic floors, guide rails, etc.), external interference (external transmitters), as well as bad positioning of the wheel electronics units with respect to the trigger and/or central antenna, a trigger signal can fail to reach the related wheel sensor or the feedback data protocol can get lost on its way to the central antenna. The control unit responds immediately by re-triggering the trigger – repeatedly if necessary – at the wheel position at which the expected protocol has failed to materialise as soon as the initiated trigger cycle from front left to rear right is completed. This concept reduces system interference and the wheel electronics units are detected much faster
Central antenna
The digital central antenna (reception frequency 315 or 433 MHz) is secured to the vehicle floor in the center tunnel between the water tubes. The signals received from the wheel electronics units are digitalized in the antenna and forwarded to the control unit via two lines (LIN bus). The digital antenna has an integrated self-diagnosis facility. This means that when a fault is detected, it is stored in the control unit fault memory and displayed on the PIWIS Tester.
Wheel electronics
The wheel electronics unit (wheel transmitter, 433 or 315 MHz) is screwed to the rim using the wheel valve. The wheel electronics unit comprises the following components: Pressure sensor, temperature sensor, roll switch, measuring and control electronics, receiver and transmitter as well as a battery.
The wheel electronics unit can be triggered and receives send requests from the TPM control unit via four trigger senders. This means that the antenna always receives only one data protocol and this comes from the wheel electronics unit that received the request to send. This gives the system added protection against crosstalk affecting other antennas. Higher transmission power is used due to the longer radio link to the central antenna. The roll switch detects whether the wheel is stationary or turning. This information is used either to start a triggered learning process for a moving wheel or to switch off the wheel electronics unit when the wheel has been stationary for a long time.
To avoid confusion with 1st generation wheel electronics, the new wheel electronics unit can be identified by the modified shape of the housing, the small air filter and the part number stamped on it. A modified data protocol ensures that the wheel electronics unit will not be detected if installed incorrectly and this is stored as a fault in the fault memory.
The following aspects of the system have been optimized by changing the system hardware and software: [...]